Alex,
I don't see that much of draw back. The only thing must be consider is the
control impedance of these signals routed on outer layer. Those signal were
routed in outer layer might get about 90 ohm control impedance and as far as
I remember, PCI signals impedance match about 50-80 ohm. This is still
possible by adding discrets to the circuitry and some smart engineering
design.
Just a reminder, routing SCSI signals on outer layers are a good move
because these signals require about 100 ohm control impedance.

> -----Original Message-----
> From: Alex Li [SMTP:alexl@ati.com]
> Sent: Monday, March 06, 2000 8:36 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: [SI-LIST] : different 4-layer board Stack up (S-P-G-S) ?
>
> Recently I saw a 4-layer mother board with 100 Mhz 128-bit memory bus.
> This board has unusual signal-power-ground-signal stack up. I talked to
> one of their engineer for this kind of arrangement. They said since most
> PC motherboard has several power plane split and on the top level there
> are a lot of components with pads. they think if they route all the
> 128-bit memory bus on the back and put it close to ground plane, they have
> much routing area and this will help to keep the signals clean.
>
> This is kind of new idea to me, does anyone see any drawback by this
> arrangement ? Will this decrease the decoupling caps performance ?
>
> Alex
>
>

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